Cool roofs are the roofs that can deliver high solar reflectance (the ability to reflect the visible, infrared and ultraviolet wavelengths of the sun, reducing heat transfer to the building) and high thermal emittance (the ability to radiate absorbed, or non-reflected solar energy).[1] The benefits associated with cool roofs include reduced cooling energy load, reduced air pollution and greenhouse gas emission, and improved human health and comfort. Cool roofs may extend the roof service life and help mitigating the urban heat island effect.[2] Cool roofs achieve cooling energy savings in hot summers but can increase heating energy load during cold winters.[3] Therefore, the net energy saving of cool roofs varies depending on climate. Without a proper maintenance program to keep the material clean, the energy savings of cool roofs can diminish over time due to albedo degradation and soiling.[4]
While it is true that cool roofs are mostly associated with white roofs, they come in a variety of colors and materials and are available for both commercial and residential buildings. Note that today's "cool roof" pigments allow metal roofing products to be EnergyStar rated in dark colors, even black. They aren't as reflective as whites or light colors, but can still save energy over other paints.
Most of the roofs in the world (including over 90% of the roofs in the United States) are dark-colored. In the heat of the full sun, the surface of a black roof can increase in temperature as much as 50 °C (90 °F), reaching temperatures of 70 to 90 °C (158 to 194 °F). This heat increase can cause negative effects on cooling energy use and environments.
Cool roofs, on the other hand, offer both immediate and long-term benefits including:
Research and practical experience with the degradation of roofing membranes over a number of years have shown that heat from the sun is one of the most potent factors that affects durability. High temperatures and large variations; seasonally or daily, at the roofing level are detrimental to the longevity of roof membranes. Reducing the extremes of temperature change will reduce the incidence of damage to membrane systems. Covering membranes with materials that reflect ultraviolet and infrared radiation will reduce damage caused by u/v and heat degradation. White surfaces reflect more than half of the radiation that reaches them, while black surfaces absorb almost all. White or white coated roofing membranes, or white gravel cover would appear to be the best approach to control these problems where membranes must be left exposed to solar radiation.[7]
If all urban, flat roofs worldwide were whitened, the reduction in carbon emissions would be 24 Gigatonnes, or equivalent to taking 300 million cars off the road for 20 years. This is based on the fact that a 1,000-square-foot (93 m2) white roof will offset 10 tons of carbon dioxide over its 20 year lifetime.[8]
While the perceived benefits are enormous, a 2011 study has found that although a reflecting roof will decrease temperatures in a single house, it will raise the "heat island effect" even more, and actually contribuite to global warming. [9] [10]
When the sunlight strikes a rooftop, most of it gets reflected back into the sky but some of its energy is absorbed into the roof system in the form of heat. Cool roofs reflect more sunlight and absorb less heat than traditional dark-colored roofs. There are two properties that are used to measure the effects of cool roofs.
Another method of evaluating coolness is the solar reflectance index (SRI), which incorporates both solar reflectance and emittance in a single value. SRI measures the roof's ability to reject solar heat, defined such that a standard black (reflectance 0.05, emittance 0.90) is 0 and a standard white (reflectance 0.80, emittance 0.90) is 100.[11]
Calculating cost savings resulting from the use of cool roofs can be done using several tools developed by federal agencies.
This tool developed by U.S. Department of Energy's Oak Ridge National Laboratory estimates cooling and heating savings for low slope roof applications with non-black surfaces.
This tool was developed by Oak Ridge National Laboratory and Lawrence Berkeley National Laboratory in order to provide industry-consensus roof savings for both residential and commercial buildings. It reports the net annual energy savings (cooling energy savings minus heating penalties) and thus is only applicable to the buildings with a heating and/or cooling system.
Cool roofs for commercial and industrial buildings fall into one of three categories: roofs made from inherently cool roofing materials, roofs made of materials that have been coated with a solar reflective coating, or green planted roofs.
Inherently cool roofs
White vinyl roofs, which are inherently reflective, achieve some of the highest reflectance and emittance measurements of which roofing materials are capable. A roof made of thermoplastic white vinyl, for example, can reflect 80 percent or more of the sun’s rays and emit at least 70% of the solar radiation that the building absorbs. An asphalt roof only reflects between 6 and 26% of solar radiation, resulting in greater heat transfer to the building interior and greater demand for air conditioning.
Coated roofs
An existing (or new) roof can be made reflective by applying a solar reflective coating to its surface. The reflectivity and emissivity ratings for over 1000 reflective roof products can be found in the CRRC (Cool Roofs Rating Council) website.
Green roofs
Green roofs provide a thermal mass layer which helps reducing the flow of heat into a building. The solar reflectance of green roofs varies depending on the plant types (generally 0.3-0.5).[12] Because of the lower solar reflectance, green roofs reflect less sunlight and absorb more solar heat than white roofs. The absorbed heat in the green roofs is trapped by the greenhouse effect and then cooled by evapotranspiration.
In some climates where there are more heating days than cooling days, white reflective roofs may not be effective in terms of energy efficiency or savings because the savings on cooling energy use can be outweighed by heating penalties during winter. According to the U.S. Energy Information Administration, 2003 Commercial Buildings Energy Consumption Survey, heating accounts for 36% of commercial buildings' annual energy consumption, while air conditioning only accounts for 8% in United States.[13] However, according to the Cool Roofs Rating Council and other sources, "The roof is an insignificant source for heat gain in winter. While cool roof owners may pay slightly more to heat their homes, this amount is usually insignificant compared to the cooling energy savings during the summer". Energy calculators generally show a yearly net savings for dark-colored roof systems in cool climates. The energy trade-off is therefore not clear cut. Additionally, higher R values for insulating materials in the roof assembly and snow covering on roofs can lessen the impact of roof surface color.
In a 2001 federal study, the Lawrence Berkeley National Laboratory (LBNL) measured and calculated the reduction in peak energy demand associated with a cool roof’s surface reflectance.[14] LBNL found that, compared to the original black rubber roofing membrane on the Texas retail building studied, a retrofitted vinyl membrane delivered an average decrease of 24 °C (43 °F) in surface temperature, an 11% decrease in aggregate air conditioning energy consumption, and a corresponding 14% drop in peak hour demand. The average daily summertime temperature of the black roof surface was 75 °C (167 °F), but once retrofitted with a white reflective surface, it measured 52 °C (126 °F). Without considering any tax benefits or other utility charges, annual energy expenditures were reduced by $7,200 or $0.07/square feet.
Instruments measured weather conditions on the roof, temperatures inside the building and throughout the roof layers, and air conditioning and total building power consumption. Measurements were taken with the original black rubber roofing membrane and then after replacement with a white vinyl roof with the same insulation and HVAC systems in place.
In July 2010, the United States Department of Energy announced a series of initiatives to more broadly implement cool roof technologies on DOE facilities and buildings across the country.[15] As part of the new efforts, DOE will install a cool roof, whenever cost effective over the lifetime of the roof, during construction of a new roof or the replacement of an old one at a DOE facility.
Energy Star is a joint program of the U.S. Environmental Protection Agency and the U.S. Department of Energy designed to reduce greenhouse gas emissions and help businesses and consumers save money by making energy-efficient product choices.
For low slope roof applications, a roof product qualifying for the Energy Star label under its Roof Products Program must have an initial solar reflectivity of at least 0.65, and weathered reflectance of at least 0.50, in accordance with EPA testing procedures.[16] Warranties for reflective roof products must be equal in all material respects to warranties offered for comparable non-reflective roof products, either by a given company or relative to industry standards.
| Slope | Min. Solar Reflectance | Min. Emittance | Min. Solar Reflectance Index |
|---|---|---|---|
| ENERGY STAR | |||
| Low, initial | 0.65 | ||
| Low, aged | 0.50 | ||
| Steep, initial | 0.25 | ||
| Steep, aged | 0.15 | ||
| Green Globes | |||
| Low Slope | 78 | ||
| Steep Slope | 29 | ||
| USGBC LEED | |||
| Low Slope | 78 | ||
| Steep Slope | 29 | ||
Cool Roof Rating Council (CRRC) has created a rating system for measuring and reporting the solar reflectance and thermal emittance of roofing products. This system has been put into an online directory of more than 850 roofing products and is available for energy service providers, building code bodies, architects and specifiers, property owners and community planners. CRRC conducts random testing each year to ensure the credibility of its rating directory.
CRRC’s rating program allows manufacturers and sellers to appropriately label their roofing products according to specific CRRC measured properties. The program does not, however, specify minimum requirements for solar reflectance or thermal emittance.
The Green Globe system is used in Canada and the United States. In the U.S., Green Globes is owned and operated by the Green Building Initiative (GBI). In Canada, the version for existing buildings is owned and operated by BOMA Canada under the brand name 'Go Green' (Visez vert).
Green Globe uses performance benchmark criteria to evaluate a building’s likely energy consumption, comparing the building design against data generated by the EPA’s Target Finder, which reflects real building performance. Buildings may earn a rating of between one and four globes. This is an online system; a building’s information is verified by a Green Globes-approved and trained licensed engineer or architect. To qualify for a rating, roofing materials must have a solar reflectance of at least 0.65 and thermal emittance of at least 0.90. As many as 10 points may be awarded for 1-100 percent roof coverage with either vegetation or highly reflective materials or both.
The U.S. Green Building Council’s Leadership in Energy and Environmental Design (LEED) rating system is a voluntary, continuously evolving national standard for developing high performance sustainable buildings. LEED provides standards for choosing products in designing buildings, but does not certify products.
Under the LEED 2009 version, to receive Sustainable Sites Credit 7.2 Heat Island Effect-Roof, at least 75% of the surface of a roof must use materials having a Solar Reflective Index (SRI) of at least 78. This criterion can also be met by installing a vegetated roof for at least 50% of the roof area, or installing a high albedo and vegetated roof in combination that meets this formula: (Area of Roof meeting Minimum SRI Roof/0.75)+(Area of vegetated roof/0.5) ≥ Total Roof Area.[17]
Examples of LEED-certified buildings with white reflective roofs are below.[18]
| Building Name | Owner | Location | LEED Level |
|---|---|---|---|
| Donald Bren School of Environmental Science & Management | University of California, Santa Barbara | Santa Barbara, California | Platinum |
| Frito-Lay Jim Rich Service Center | Frito-Lay, Inc. | Rochester, New York | Gold |
| Edifice Multifunction | Travaux Public et Services Gouvernementaux Canada | Montreal, Quebec | Gold |
| Seattle Central Library | City of Seattle | Seattle, Wash. | Silver |
| National Geography Society Headquarters Complex | National Geographic Society | Washington, D.C. | Silver |
| Utah Olympic Oval | Salt Lake City Olympic Winter Games 2002 Organizing Committee | Salt Lake City, Utah | Certified |
| Premier Automotive Group North American Headquarters | Ford Motor Company | Irvine, California | Certified |
This project is co-financed by the European Union in the framework of the Intelligent Energy Europe Programme.
The aim of the proposed action is to create and implement an Action Plan for the cool roofs in EU. The specific objectives are: to support policy development by transferring experience and improving understanding of the actual and potential contributions by cool roofs to heating and cooling consumption in the EU; to remove and simplify the procedures for cool roofs integration in construction and building’s stock; to change the behaviour of decision-makers and stakeholders so to improve acceptability of the cool roofs; to disseminate and promote the development of innovative legislation, codes, permits and standards, including application procedures, construction and planning permits concerning cool roofs.[19] The work will be developed in four axes, technical, market, policy and end-users.
In tropical Australia, zinc-galvanized (silvery) sheeting (usually corrugated) do not reflect heat as well as the truly "cool" color of white, especially as metallic surfaces fail to emit infrared back to the sky.[20] European fashion trends are now using darker-colored aluminium roofing, to pursue consumer fashions.
An urban heat island occurs where the combination of heat-absorbing infrastructure such as dark asphalt parking lots and road pavement and expanses of black rooftops, coupled with sparse vegetation, raises air temperature by 1 to 3°C higher than the temperature in the surrounding countryside.[21]
Green building programs advocate the use of cool roofing to mitigate the urban heat island effect and the resulting poorer air quality (in the form of smog) the effect causes. By reflecting sunlight, light-colored roofs minimize the temperature rise and reduce cooling energy use and smog formation. A study by LBNL showed that, if strategies to mitigate this effect, including cool roofs, were widely adopted, the Greater Toronto metropolitan area could save more than $11 million annually on energy costs.[22]
Cool Cars (solar reflective cars) reflect more sunlight than dark cars, reducing the amount of heat that is transmitted into the car’s interior. Therefore, it helps decreasing the need for air conditioning, fuel consumption, and emissions of greenhouse gases and urban air pollutants[23].
In California, over 95% of the cars and small trucks are equipped with air conditioners, burning fossil fuels and producing greenhouse gas emissions. The Heat Island Group at Lawrence Berkeley National Laboratory (LBNL) conducted the cool cars project since 2010, sponsored by the California Energy Commission (CEC), with the goal of reducing air conditioning usage of cars by lowering cabin air temperatures.